A major challenge in the study of mammalian brain development is to identify the extracellular ligands and receptors that direct the assembly of the complex wiring pattern of the brain. A recent modification of the gene trap technique in embryonic stem cells, the "secretory trap", allows the systematic trapping of genes encoding ligands and receptors, and the generation of mice harboring mutations in these genes, providing an important tool to elucidate the functions of these genes. However, the problem with studying brain wiring is that it can be extremely difficult to identify changes in wiring resulting from experimental perturbation because of the difficulty of tracing the axonal projections of neurons. In particular, it is usually extremely difficult to identify wiring defects in mutant mice, whether the mutation arose spontaneously or was generated by gene targeting or insertional mutagenesis. We have conceived a further modification of the gene trap technique that should now dramatically facilitate the identification and characterization of receptors involved in wiring the mammalian nervous system. In our method, a histochemical axonal marker is targeted to neurons that normally express the trapped gene. This enables direct visualization of the connections made by these neurons, and direct visualization of wiring defects in homozygous mutant animals, through a simple histochemical stain. In this way the role of the trapped gene in brain wiring can be rapidly assessed. Of equal importance, and of great benefit to the Neuroscience community at large, the method will also simultaneously result in the generation of a bank of mice expressing the histochemical marker in different populations of axons, which will provide an important tool for researchers interested in elucidating the normal pattern of neuronal connections in the mammalian brain. A request is made here to support the isolation of a large number of secretory trap ES cell lines using this modified vector. It is proposed that 80 of these lines per year will be put through the germ line for expression and phenotypic analysis to identify those involved in wiring the nervous system, and to provide a resource of mice expressing the axonal marker in different populations of neurons for use by the Neuroscience community. We expect that this method will help greatly accelerate the pace of discovery of mechanisms that direct the wiring of the mammalian nervous system.